Rear projection screens transmit an image projected onto the rear of the screen into a viewing space. A rear projection screen may be a sheetlike optical device with a relatively thin viewing layer that is placed at an image surface of the projector. Examples of rear projection displays are disclosed in PCT WO 99/064927, PCT WO 99/13378 and EP 783 133.
Projection screens are often constructed of rigid materials such as glass or a semi-rigid, transparent polymeric material. As a result, it is not possible to conveniently roll an inflexible projection screen in the fashion that a diploma or map may be rolled as it will damage the screen. Examples of a typical prior art rigid projection screens comprise the DA series of diffusion screens from DA-Lite. Rigid projection screens are described in U.S. Pat. No. 6,163,402.
Other rear projection optical systems may include a fresnel lens and/or a lenticular lens or sheet. Examples of such systems are disclosed in U.S. Pat. Nos. 3,712,707; 3,872,032; 4,379,617; 4,418,986; 4,468,092 and 4,509,823.
Front projection systems are also known in the art. They comprise a projector designed to project an image on a surface (e.g. the wall of a conference room or a screen). Overhead projectors are an example of a front projection system. Rear projection screens provide several advantages over front projection screens. With a rear projection screen, the presenter cannot cast a shadow on the image and the projection equipment may be hidden from view (which also helps mask any acoustical background noise emanating from electrical components). Front projection systems have less capacity to absorb ambient light than rear projection systems.
One known rear projection screen comprises a thin, light diffusing layer (frosted or translucent glass) constructed by etching, sandblasting or otherwise roughening a smooth glass surface. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent tend to strongly reflect ambient light incident on the front, viewing side, resulting in fading or washout of the projected image. As a result, this rear projection screen is sensitive to ambient lighting conditions.
U.S. Pat. No. 4,323,301 discloses a collapsible and portable screen assembly that is said to be able to act as a rear or front projection screen. The assembly includes an inflatable frame and a flexible sheet of screen material.
U.S. Pat. No. 2,378,252 discloses a rear projection screen comprising an array of closely packed glass beads associated with a transparent support and a light absorbing layer. The glass beads perform lens-like functions to collect light projected from the rear of the screen and focus it to relatively small spots near the area where the beads contact the support. The glass beads contact the transparent substrate and thereby exclude most of the light absorbing material at the contact area location between the glass beads and support. The light absorbing layer absorbs ambient light incident on the front surface of the support. As a result, the front side of the screen appears dark, except for the light transmitted through the glass beads.
Rear projection screens with glass beads are also disclosed in U.S. Pat. Nos. 5,563,738 and 5,781,344. Additional beaded screens and methods of making such screens are disclosed in commonly assigned patent applications PCT WO 99/50710 and PCT WO 98/45753.
Televisions are used for advertising in malls, showrooms and product exhibitions. Many televisions used for this type of advertising are permanently installed or very large and heavy and, as a result, are not portable.
Projection systems as opposed to televisions are also used for advertising in malls, showrooms, and exhibitions. Rear projection systems are one such example. They consist of at least a projection device (e.g. LCD) and a rear projection screen. Typically, the projector is installed on a floor or, alternatively, the ceiling. Installation of floor or ceiling mounted systems requires significant resources. Complicated or time consuming installation procedures contribute to the cost of the system. While rear projection systems are available that are self-contained units, such self-contained rear projection systems typically include bulky framework and housing elements that render the systems difficult to reposition. Rear projection systems with large screens also tend to be quite heavy, further contributing to difficulties associated with transporting and installing the optical system.
Another rear projection system is capable of providing holographic images. An example of such a screen is the dnp Holo Screen™ available from DAINIPPON PRINTING Co., Ltd. of Japan or its associated organization in Denmark. This system includes a thin hologram film laminated to a transparent acrylic plate. The hologram film consists of four holograms that are tiled together. The hologram film will only respond to light rear projected from thirty five degrees. This film is very selective about the direction from which it receives the light, thereby complicating installation. In a room with at least some ambient background lighting, this screen remains transparent where there is no projected image.
Other examples of a rear projection screen are the Hitachi On-Glass Projection System 40″/60″ and the AirSho™ 40″/60″ Display Projection System, both available from Hitachi of Brisbane Calif. A typical screen is constructed from a photopolymer resin that refracts light incident at the thirty five degree angle. A 0.3 μm to 2.0 μm pitch is applied to the screen. The screen is transparent and very expensive. A permanent adhesive is used to attach the screen to a store window. The screen lacks an efficient ambient light absorbing layer, hence it is susceptible to washout in brightly lit conditions. As a result, these screens require very bright (1700 ANSI Lumen and above) projectors. These bright projectors are expensive to purchase. They are also expensive to maintain due to short lamp lives.
A permanently installed optical system runs the risk of becoming commonplace. This is a problem as purchasing decisions are often made at the point of purchase. Over time, customers that frequent a particular location may take a permanently installed optical system for granted. A static display can lose its splendor and become less valuable.